1. Field of the Invention
The invention relates to generators of electromagnetic, light or infrared waves having plasma confining modes and, more particularly to generators of such waves embodying non confining to confining mode converters.
Confinement of plasmas is of prime importance in research on coherent X-ray emission or the approach of thermonuclear fusion by focusing high power laser beams on to a fusion material.
In official wave terminology, the wave mode is followed by two subscripts. The first denotes the number of spatial periods in the azimuthal direction and the second denotes the number of spatial periods in the radial direction. In the present specification, we shall never give the second subscript since, from this point of view, the mode will always be near the fundamental conditions, the proportion of harmonics being relatively low and varying with the distance from the axis when the intensity of the wave is varied.
2. Description of Prior Art
Confining mode waves are defined in U.S. Pat. application Ser. No. 863,235 filed Dec. 11, 1977 in the name of the present Applicant.
The term "plasma confining waves" or simply "confining waves" is used for electromagnetic waves having an electric field which is zero at at least one point along the propagation axis. Confining waves have the property of collecting electrons at confinement points where the electric field is zero. The confining force driving the electrons towards the points of confinement is proportional to the gradient of the square of the modulus of the electric field. This confining force contains an electric component and a magnetic component; the magnetic component can be neglected in certain cases.
TE.sub.0, TE.sub.2, TM.sub.2 modes and, more generally, TEM.sub.q modes are confining modes, the electric field being zero at all the points on their axes of propagation. In the case of the TE.sub.0, TE.sub.2 and TM.sub.2 modes, the confining potential near the axis is proportional to the square of the distance from the axis. In the case of TEM.sub.q modes with q greater than 2, the confining potential near the axis is proportional to the 2(q-1).sup.th power of the distance from the axis. As it will be explained later, the TM.sub.0 wave is also a confining mode wave when it is stationary.
These confining propagation modes are used in combination with an axial magnetic field, which is designed for axially pinching the electrons converging towards the axis, as we have already shown in the aforementioned patent application and will show in a more general manner hereinafter.
Circular-polarized waves having a positive or negative azimuthal phase shift have the property of confining plasmas. If we limit ourselves to first-order azimuthal phase shifts, positively phase-shifted circular-polarized waves are mixtures of TE.sub.0 and TM.sub.0 modes in phase quadrature and negatively phase-shifted circular polarized waves are two TE.sub.2 modes in phase quadrature and in space quadrature. In the case of TE.sub.0 and TE.sub.2 modes, the electrical field has no component outside the phase planes. The TM.sub.0 mode has an axial component which is negligible when the beam used has a large diameter, of the order of 10.sup.4 times the wavelength. The axial component of the electric field of the TM.sub.0 mode becomes important when focusing occurs; it may reach the same order of magnitude as the radial field in the phase plane or may become even larger. Consequently, in the case of circular polarized waves having a first-order poritive azimuthal phase shift, the square of the electric field modulus is not zero along the propagation axis. If the square of the electric field modulus obtained on a circle of radius .sigma. has a given maximum value, the confining potential difference is much less for positively phase-shifted waves than for negatively phase-shifted waves.
As already said, the TM.sub.0 wave is not confining except when steady, since in that case its axial electric field cancels out at each half wavelength. When the power of a laser is focused on to a solid material, producing a plasma, it is practivally impossible to obtain a steady wave since the absorbed energy progressively reduces the wave intensity and the energy is reflected in accordance with a diagram which is very different from that given by a plane mirror.
For these reasons, when attempting to obtain controlled thermonuclear fusion, it is preferable to use generators of laser beams having a circular-polarized confining mode which has a negative azimuthal phase shift and not a positive phase-shift.
The maximum power of neodymium lasers is proportional to the diameter on the doped glass cylindrical bars. In practice, the diameter does not exceed 10-12 cm since, at larger diameters, the light of the floodlamps energizing the laser is exclusively absorbed at the periphery and does not energize the interior of the laser bar. This disadvantage is obviated by using cylindrical sleeves of doped glass which present a larger area to the floodlamps for a given cross-section area available for laser beam propagation. The thickness of the sleeve thus corresponds to an optimum thickness for penetration of the pumping wave. The laser wave emitted by an aforementioned sleeve propagates in the TM.sub.0 mode, which has a pattern of revolution adapted to the structure of the cylindrical sleeve.
It results from the foregoing that the TM.sub.0 mode is a poor mode as regards to confinement and a convenient mode as regards to laser amplification.
Conventional laser beam generators produce rectilinear-polarized electromagnetic waves corresponding to the TE.sub.1 propagation mode. The aforementioned patent application described means for converting rectilinear-polarized laser beam generators into confining mode laser beam generators. Two embodiments of mode converters were disclosed. The first kind of mode converters comprises sector-shaped half-wave plates disposed edge to edge on a transparent support so that their rapid (or slow) axes form broken hyperbolic or parabolic lines. The second kind of mode converters comprises transparent plates, one surface of which is helicoidal or forms a spiral staircase. In the abovementioned patent application, the mode converters convert rectilinear-polarized input wave beam into negatively azimuthally phase shifted circular polarized output wave beam. For doing this, the converters rotate the polarization vector of the input wave through an angle -.phi. around the beam axis at each point of azimuth +.phi..